“Battery capacity” is a measure (typically in ampere-hours) of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The battery capacity represents the maximum amount of energy that can be extracted from the battery under certain specified conditions. The effective capacity of the battery (battery “state of health”), however, can vary significantly from the “nominal” rated capacity.
The effective storage capacity of the battery diminishes with age and undergoes irreversible damage. This damage is caused by corrosion and other irreversible chemical processes. Aging of the internal battery components contributes to the damage as well. The plates of the cells corrode, and as the plates corrode, their operational surface area diminishes and the electrolyte undergoes chemical changes, causing them both to be less chemically reactive. The changes reduce the volume of reactive components in the cell, reducing the charge capacity of the cell. It also increases the internal resistance of the cell, as the corrosion products inhibit the free flow of electrons through the plates. Each charge/discharge cycle of the battery also has a similar effect but at an accelerated rate. As a result, as the battery ages and deteriorates, the effective capacity of the battery decreases, reducing the time the battery can supply current to a device. In addition, the voltage level drops off faster, applying a decreasing voltage to the device.
Conventional methods for measuring the effective capacity of the battery are inefficient and prone to errors because many methods require removal of the battery from the device. Other methods require a sense resistor for measuring the current over a known time frame, which occupies a large area, is costly, and decreases performance.